The socketserver module simplifies the task of writing network servers.

There are four basic server classes: TCPServer uses the Internet TCP
protocol, which provides for continuous streams of data between the client and
server. UDPServer uses datagrams, which are discrete packets of
information that may arrive out of order or be lost while in transit. The more
infrequently used UnixStreamServer and UnixDatagramServer
classes are similar, but use Unix domain sockets; they’re not available on
non-Unix platforms. For more details on network programming, consult a book
such as
W. Richard Steven’s UNIX Network Programming or Ralph Davis’s Win32 Network
Programming.

These four classes process requests synchronously; each request must be
completed before the next request can be started. This isn’t suitable if each
request takes a long time to complete, because it requires a lot of computation,
or because it returns a lot of data which the client is slow to process. The
solution is to create a separate process or thread to handle each request; the
ForkingMixIn and ThreadingMixIn mix-in classes can be used to
support asynchronous behaviour.

Creating a server requires several steps. First, you must create a request
handler class by subclassing the BaseRequestHandler class and
overriding its handle() method; this method will process incoming
requests. Second, you must instantiate one of the server classes, passing it
the server’s address and the request handler class. Finally, call the
handle_request() or serve_forever() method of the server object to
process one or many requests.

When inheriting from ThreadingMixIn for threaded connection behavior,
you should explicitly declare how you want your threads to behave on an abrupt
shutdown. The ThreadingMixIn class defines an attribute
daemon_threads, which indicates whether or not the server should wait for
thread termination. You should set the flag explicitly if you would like
threads to behave autonomously; the default is False, meaning that
Python will not exit until all threads created by ThreadingMixIn have
exited.

Server classes have the same external methods and attributes, no matter what
network protocol they use.

Note that UnixDatagramServer derives from UDPServer, not from
UnixStreamServer — the only difference between an IP and a Unix
stream server is the address family, which is simply repeated in both Unix
server classes.

Forking and threading versions of each type of server can be created using the
ForkingMixIn and ThreadingMixIn mix-in classes. For instance,
a threading UDP server class is created as follows:

classThreadingUDPServer(ThreadingMixIn,UDPServer):pass

The mix-in class must come first, since it overrides a method defined in
UDPServer. Setting the various attributes also change the
behavior of the underlying server mechanism.

To implement a service, you must derive a class from BaseRequestHandler
and redefine its handle() method. You can then run various versions of
the service by combining one of the server classes with your request handler
class. The request handler class must be different for datagram or stream
services. This can be hidden by using the handler subclasses
StreamRequestHandler or DatagramRequestHandler.

Of course, you still have to use your head! For instance, it makes no sense to
use a forking server if the service contains state in memory that can be
modified by different requests, since the modifications in the child process
would never reach the initial state kept in the parent process and passed to
each child. In this case, you can use a threading server, but you will probably
have to use locks to protect the integrity of the shared data.

On the other hand, if you are building an HTTP server where all data is stored
externally (for instance, in the file system), a synchronous class will
essentially render the service “deaf” while one request is being handled –
which may be for a very long time if a client is slow to receive all the data it
has requested. Here a threading or forking server is appropriate.

In some cases, it may be appropriate to process part of a request synchronously,
but to finish processing in a forked child depending on the request data. This
can be implemented by using a synchronous server and doing an explicit fork in
the request handler class handle() method.

Another approach to handling multiple simultaneous requests in an environment
that supports neither threads nor fork() (or where these are too expensive
or inappropriate for the service) is to maintain an explicit table of partially
finished requests and to use select() to decide which request to work on
next (or whether to handle a new incoming request). This is particularly
important for stream services where each client can potentially be connected for
a long time (if threads or subprocesses cannot be used). See asyncore
for another way to manage this.

The address on which the server is listening. The format of addresses varies
depending on the protocol family; see the documentation for the socket module
for details. For Internet protocols, this is a tuple containing a string giving
the address, and an integer port number: ('127.0.0.1',80), for example.

The size of the request queue. If it takes a long time to process a single
request, any requests that arrive while the server is busy are placed into a
queue, up to request_queue_size requests. Once the queue is full,
further requests from clients will get a “Connection denied” error. The default
value is usually 5, but this can be overridden by subclasses.

This function is called when the timeout attribute has been set to a
value other than None and the timeout period has passed with no
requests being received. The default action for forking servers is
to collect the status of any child processes that have exited, while
in threading servers this method does nothing.

Calls finish_request() to create an instance of the
RequestHandlerClass. If desired, this function can create a new process
or thread to handle the request; the ForkingMixIn and
ThreadingMixIn classes do this.

Must return a Boolean value; if the value is True, the request will
be processed, and if it’s False, the request will be denied. This
function can be overridden to implement access controls for a server. The
default implementation always returns True.

This function must do all the work required to service a request. The
default implementation does nothing. Several instance attributes are
available to it; the request is available as self.request; the client
address as self.client_address; and the server instance as
self.server, in case it needs access to per-server information.

The type of self.request is different for datagram or stream
services. For stream services, self.request is a socket object; for
datagram services, self.request is a pair of string and socket.
However, this can be hidden by using the request handler subclasses
StreamRequestHandler or DatagramRequestHandler, which
override the setup() and finish() methods, and provide
self.rfile and self.wfile attributes. self.rfile and
self.wfile can be read or written, respectively, to get the request
data or return data to the client.

importsocketserverclassMyTCPHandler(socketserver.BaseRequestHandler):""" The RequestHandler class for our server. It is instantiated once per connection to the server, and must override the handle() method to implement communication to the client. """defhandle(self):# self.request is the TCP socket connected to the clientself.data=self.request.recv(1024).strip()print("{} wrote:".format(self.client_address[0]))print(self.data)# just send back the same data, but upper-casedself.request.sendall(self.data.upper())if__name__=="__main__":HOST,PORT="localhost",9999# Create the server, binding to localhost on port 9999server=socketserver.TCPServer((HOST,PORT),MyTCPHandler)# Activate the server; this will keep running until you# interrupt the program with Ctrl-Cserver.serve_forever()

An alternative request handler class that makes use of streams (file-like
objects that simplify communication by providing the standard file interface):

classMyTCPHandler(socketserver.StreamRequestHandler):defhandle(self):# self.rfile is a file-like object created by the handler;# we can now use e.g. readline() instead of raw recv() callsself.data=self.rfile.readline().strip()print("{} wrote:".format(self.client_address[0]))print(self.data)# Likewise, self.wfile is a file-like object used to write back# to the clientself.wfile.write(self.data.upper())

The difference is that the readline() call in the second handler will call
recv() multiple times until it encounters a newline character, while the
single recv() call in the first handler will just return what has been sent
from the client in one sendall() call.

importsocketserverclassMyUDPHandler(socketserver.BaseRequestHandler):""" This class works similar to the TCP handler class, except that self.request consists of a pair of data and client socket, and since there is no connection the client address must be given explicitly when sending data back via sendto(). """defhandle(self):data=self.request[0].strip()socket=self.request[1]print("{} wrote:".format(self.client_address[0]))print(data)socket.sendto(data.upper(),self.client_address)if__name__=="__main__":HOST,PORT="localhost",9999server=socketserver.UDPServer((HOST,PORT),MyUDPHandler)server.serve_forever()

This is the client side:

importsocketimportsysHOST,PORT="localhost",9999data=" ".join(sys.argv[1:])# SOCK_DGRAM is the socket type to use for UDP socketssock=socket.socket(socket.AF_INET,socket.SOCK_DGRAM)# As you can see, there is no connect() call; UDP has no connections.# Instead, data is directly sent to the recipient via sendto().sock.sendto(bytes(data+"\n","utf-8"),(HOST,PORT))received=str(sock.recv(1024),"utf-8")print("Sent: {}".format(data))print("Received: {}".format(received))

The output of the example should look exactly like for the TCP server example.

To build asynchronous handlers, use the ThreadingMixIn and
ForkingMixIn classes.

An example for the ThreadingMixIn class:

importsocketimportthreadingimportsocketserverclassThreadedTCPRequestHandler(socketserver.BaseRequestHandler):defhandle(self):data=str(self.request.recv(1024),'ascii')cur_thread=threading.current_thread()response=bytes("{}: {}".format(cur_thread.name,data),'ascii')self.request.sendall(response)classThreadedTCPServer(socketserver.ThreadingMixIn,socketserver.TCPServer):passdefclient(ip,port,message):sock=socket.socket(socket.AF_INET,socket.SOCK_STREAM)sock.connect((ip,port))try:sock.sendall(bytes(message,'ascii'))response=str(sock.recv(1024),'ascii')print("Received: {}".format(response))finally:sock.close()if__name__=="__main__":# Port 0 means to select an arbitrary unused portHOST,PORT="localhost",0server=ThreadedTCPServer((HOST,PORT),ThreadedTCPRequestHandler)ip,port=server.server_address# Start a thread with the server -- that thread will then start one# more thread for each requestserver_thread=threading.Thread(target=server.serve_forever)# Exit the server thread when the main thread terminatesserver_thread.daemon=Trueserver_thread.start()print("Server loop running in thread:",server_thread.name)client(ip,port,"Hello World 1")client(ip,port,"Hello World 2")client(ip,port,"Hello World 3")server.shutdown()